The present invention relates to the field of aircraft turbine blades and more particularly the cooling of ceramic blades.
Ceramic blade outer airseal life is a perennial problem in the first stage of high temperature turbines. Typical ceramic airseal failure is characterized by either cracking or partial spalling, resulting in an increased turbine tip clearance which produces a high leakage flow, reducing turbine efficiency. Such failure can be alleviated by the incorporation of a film cooling of the upper surface of the ceramic blade portion.
FIG. 1 illustrates a typical prior art ceramic airseal cooling design for a turbine tip. The upper surface 11 is the hottest portion of the ceramic body 1 and is cooled by the impingement of air illustrated by arrows 5 at the lower surface of the support substrate of the ceramic body. The cooling air flows through air ducts 7 and 9 to cool the lower surface 13 of ceramic body 1. Since the ceramic layer has a low thermal conductivity, this cooling technique produces a very hot ceramic upper surface 11 and a relatively cold lower inner surface, resulting in a high thermal gradient across the ceramic layer, to induce a high thermal stress tending to weaken the blade.
The teaching of passing cooling air through parallel conduits embedded in turbine blade portions is old in the art. The cooling air passes through, for example, honeycombed passages in U.S. Pat. 3,172,621 to Erwin, and forms a cooling air film upon the surface of the airfoil containing the cooling air honeycombed passages. U.S. Pat. No. 4,384,823 to Graham et al illustrates the concept of providing an array of cooling air tubes which transmit air to surfaces of a turbine blade. Other U.S. Pat. Nos. such as 4,684,322 and 4,249,291, illustrate the teaching of embedding cooling tubes within the body of turbine blade portions to cool surfaces thereof. It is the principle object of the present invention to provide a method of producing an array of cooling air passages embedded within a ceramic blade portion to be cooled which is believed to be more suitable than the approaches of the prior art.